Refractory gold ores are ores that are generally not amenable to standard cyanidation gold extraction methods as known in the art. Conventional straight cyanidation gold extraction methods typically facilitate less than about 50% gold extraction when used on refractory gold ores. Gold-bearing ores are often refractory because of their content of organic carbonaceous and/or sulfidic matter which inhibits or substantially reduces extraction of gold using these conventional cyanidation techniques. Such refractory ores may be found in Nevada and other states in the United States, as well as in other countries throughout the world. It is not completely understood, in all instances, why the sulfidic matter and the organic carbonaceous materials cause such ores to be refractory, but the phenomenon and the resistance to conventional cyanidation extraction is well known in the gold mining industry. The gold may also be encapsulated in mineral entities of the ore which are not attacked by conventional cyanidation processes.
A number of processes have been developed to treat refractory gold ores to make the ores more amenable to conventional gold recovery methods. For example, some ores which owe their refractory nature to their content of organic carbonaceous matter can be handled efficiently by using the process taught in commonly assigned U.S. Pat. No. 4,289,532 to Matson et al. This process treats carbonaceous ores with an oxidation step utilizing chlorine to oxidize the carbonaceous matter prior to a cyanide leach step. Although this process was intended primarily to treat ores containing carbonaceous matter, the process has also been found effective in treating gold ores containing low levels of sulfidic compounds in addition to carbonaceous matter. The sulfidic compounds are oxidized by contact with chlorine. However, when high sulfide concentrations are present in the ore, excessive quantities of chlorine are required and the process becomes more costly. Other treatment techniques, such as autoclaving or roasting, may be necessary for ores containing high concentrations of sulfidic compounds.
Two processes utilizing autoclaving to treat refractory ores are described in U.S. Pat. No. 4,552,589 to Mason et al and commonly assigned U.S. Pat. No. 4,738,718 to Bakshani et al. Numerous patents teach roasting processes for handling refractory ores. One such patent is commonly assigned U.S. Pat. No. 4,919,715 to Smith et al. The process of that patent is specifically directed to treating refractory sulfidic and carbonaceous ores.
In addition to the ore treatment steps described, the above-noted processes have the disadvantage of requiring cyanide leach step to recover the gold from the treated ore. While cyanide leaching has been an extremely effective means of recovering gold from most ores, concern about the potential toxic effects of cyanide has been increasing in recent years. For example, tightened tolerance levels of the cyanide content of ground water and discharge water have been set in several states and the trend toward such tightened standards will probably continue. Because of the increased concern over the toxicity of cyanide, considerable research has been directed in recent years toward finding an effective substitute for cyanide in gold leaching operations without sacrificing gold recoveries.
The potential toxicity of cyanide is not the only driving force for research in developing alternative gold leaching agents. As easily processed gold ores become exhausted, refractory ores of various types will have to be handled. Because of the chemical characteristics of some of these refractory ores, cyanide is not always the most effective leaching agent.
Two leaching compounds, acidic thiourea and thiosulfate, have received considerable attention in the gold industry. The two compounds have been found effective for leaching some ores, and ineffective, or uneconomical, for leaching most other ores.
Some attention has also been redirected to chlorine which was the most widely used gold leaching agent prior to being displaced by the more efficient cyanide at the turn of the century. For example, commonly assigned U.S. Pat. No. 4,723,998 to O'Neil uses a simultaneous chlorine leaching and ion exchange resin adsorption procedure to recover gold from carbonaceous ores.
None of the alternative leaching agents and methods discussed above have been sufficiently effective in terms of cost efficiency and gold recoveries to replace cyanide leaching as the universal leaching agent in the gold industry.
There is obviously still a need in the gold mining industry for an effective and efficient leaching agent which can, at least partially, replace or supplement cyanide leaching. The present invention is directed to a new gold recovery method which is effective in recovering gold values from many gold ores, particularly from many refractory gold ores. The gold values are recovered from the gold ore using a pretreatment which renders the gold content of ore amenable to leaching with sulfurous acid.